The present invention relates to a design for a nozzle and method of making the nozzle for use in an alloy production process, and particularly in processing noncontaminated molten titanium or titanium alloys.
2. Description of Related Art
It is widely recognized that one of the most important and urgent areas of materials research in the coming decade is the advancement of materials processing technology for a new generation of materials including metals and metal alloys. As an example, eliminating or substantially reducing the material impurities and eliminating or substantially reducing the presence of defects in fabricated parts or components are considered the major bottlenecks in improving the quality of the high performance aircraft engines to be built in this decade and beyond.
Efforts have heretofore concentrated on producing high quality metal powders to be employed in the fabrication of components, and the concentration on production of high quality powders from which components may be made is regarded as a major step in making "clean" materials for parts or components. The production of titanium and/or titanium alloys in powder or ingot form is of special significance in the aircraft engine field, due to the importance of the titanium and its alloys in designing and producing improved engine components. Notwithstanding the effort expended in developing processes or methods to produce high quality metal powders, a serious problem persists with respect to the production of high quality titanium and titanium alloys in that the high level of chemical reactivity of liquid titanium yields or tends to yield unacceptable levels of impurities in the intermediate forms, such as powders, or in the end product.
Because of the high reactivity of liquid titanium, the melting of the titanium or Ti alloy and discharging of the liquid titanium or Ti alloy are generally done in a technique known in the art as cold hearth or skull melting. An example of this technique is described in U.S. Pat. No. 4,654,858, issued to Rowe, and assigned to the assignee of the present application. Other skull melting configurations have also been disclosed in the art, and all of these may be characterized as having a crucible which retains the molten titanium, the crucible being made of a material other than titanium, and, in the "bottom pouring" embodiments, a discharge nozzle, also likely to be made of a material other than titanium. The skull melting technique attempts to avoid the problem of a reaction occurring between the liquid titanium and the crucible and nozzle materials by developing a skull of solid titanium covering the internal surfaces of the crucible and nozzle. The term "continuous skull nozzle process" will be used herein to refer to processes of this type in general.
While continuous skull nozzle processes have been in use in the art for a number of years, problems remain in such processes, particularly those in which-an elongated bottom discharge nozzle is employed (as compared with an orifice as depicted in the above-identified '858 patent), in that the formation and control of a stable skull inside the nozzle has proven to be a major hurdle in the development of consistent, dependable processes for melting and discharging the liquid metal from the crucible. The two principal problems experienced with skull formation in the nozzle are skull "freeze-off" and skull "melt-away". Freeze-off of the skull prevents the continued flow of the liquid metal out of the crucible to a further apparatus, such as a melt spinning device or continuous ingot casting device. Melt-away of the skull leaves the nozzle material exposed to react with the liquid titanium or alloy, which is likely to cause rapid deterioration of the nozzle by way of either chemical reaction or physical erosion, resulting in contamination of the liquid metal by impurities from the nozzle.
Prior attempts to control skull freeze-off or otherwise stabilize the skull geometry in the nozzle have all suffered from disadvantages which have ultimately rendered the proposed solutions ineffective, impractical, and in some instances, undesirable. In one such proposed solution, local induction heating applied to the skull at the nozzle was attempted as a means for preventing nozzle freeze-off from occurring. This approach proved to be ineffective at providing the necessary heat penetration required for maintaining a molten stream at the center of the nozzle, due to the skin effect which concentrates the heat generated at the outer portions of the nozzle and skull. The skin effect of the induction heating actually has a counterproductive effect in that most of the heat generation is concentrated at the outer skin, where a layer of solidified skull is required to be maintained.
The concept of a magnetic levitation nozzle has been propounded as an alternative approach to providing a physical crucible and nozzle structure, thereby eliminating contact between the containment or confinement means and the liquid titanium or alloy thereby preventing any chemical reaction from taking place. Because of the limited strength of the magnetic force, the potential for replacing the skull crucible and nozzle with a levitation nozzle, in view of the current level of technology, shows almost no promise.
The levitation nozzle approach has been proposed for use on a more limited basis to confine the melt stream only. In this approach, an induction coil would be used to confine the melt stream by generating a magnetic field to induce a thin layer of "body force" on the surface of the melt stream, the force having substantially the same effect as creating a positive hydrostatic pressure at the melt stream. The purpose of this type of levitation confinement is to control the flow rate and diameter of the liquid metal melt stream, without specifically dealing with the problem of maintaining a stable skull geometry in the nozzle.
Even in this more limited approach the levitation nozzle is unattractive due to problems intrinsic to the design of the induction coil, and due to problems in the application of this technology to confining the melt stream, such as the alignment of the coil, the stability of the induced current, the electromagnetic field interference and coupling, the complicated coil design, and problems with melt stability, asymmetry and splash. Further, since a crucible and nozzle would still be fundamental components in a system employing levitation to control the diameter of the melt stream, the complicated coupling and interaction between the levitation nozzle and the overall system would require tremendous experimental effort to validate the concept. Simplified experiments are not likely to adequately address the interactions among the levitation force, the nozzle size, and the formation, growth and control of the skull.
One proposed solution to achieving a desired steady-state solidified skull at the nozzle region in a continuous skull nozzle process has been set forth in copending U.S. patent application Ser. No. 07/552,980, filed Jul. 16, 1990, and assigned to the assignee of the present application. That application is hereby incorporated by reference. In that application, a systematic investigation of the continuous skull nozzle process was undertaken, and a process window was identified or defined such that a control strategy could be implemented so as to maintain a steady-state solidified skull in the nozzle region, which would not be subject to freeze-off or melt-away of the skull. A method for controlling the molten metal flow using a pressure differential between the interior and exterior of the crucible is proposed in that application as a means for governing the process to maintain operation within the defined process window.
Even with the process window approach in hand, a major hurdle is present in continuous skull nozzle processing in that the flow radius at the critical nozzle region will generally be too small to allow a stable solidified layer to be formed and maintained unless the cooling at the nozzle region is significant. No solution to this particular aspect of the continuous skull nozzle process has heretofore been propounded which would readily enable operation of the process within the defined process window resulting in the maintenance of a steady-state solidified skull.
It is therefore a principal object of the present invention to provide a design for a nozzle assembly which will allow suitable process controls to be employed for maintaining a stable solidified skull layer inside the nozzle.
It is another important object of the present invention to provide a method for constructing a nozzle which will permit operation of the process comfortably within the process window for maintaining a stable solidified skull layer inside the nozzle.